Close encounters of the stellar kind

A star flashed past our solar system about 70,000 years ago. Did our ancestors even notice it?
Schols's star, an artist's conception

An artist’s conception of Scholz’s star during its flyby of the solar system 70,000 years ago. The Sun (left, background) would have appeared as a brilliant star. The pair is now about 20 light years away. Credit: Michael Osadciw/University of Rochester.

P Rajendran

About 70,000 years ago two stars in a free spin careened across the edge of our solar system, little more than half a light year away.

When first discovered by Ralf-Dieter Scholz of the Leibniz-Institut für Astrophysik Potsdam in Germany in 2013, the pair had already waltzed 20 light years away.

The pair – nicknamed Scholz star – was hard to find in the first place because it is small. The larger unit, a red dwarf, is just about 0.09 the mass of the sun; the smaller unit, a brown dwarf is about 0.06 times as big as the sun. Though about 65 times as big as Jupiter, it lacks stellar gravitas since its low mass means it cannot generate enough heat and pressure to produce light-generating nuclear fusion. That is when hydrogen is compressed enough to ultimately form helium.

Scholz’s star today (circled in red) Courtesy:
Strasbourg Astronomical Data Center

Eric Mamajek at the University of Rochester and a team including researchers from Europe, Chile and South Africa, had seen that the Scholz star moved very slowly and tried to ascertain whether its path was taking it towards or away from the solar system. They did 10,000 simulations of the possible path it could take, allowing for their location, speed, distance and the gravitational effects from surrounding objects. They found 98 percent of the simulations showed it swinging through the Oort cloud, an distant zone of icy debris surrounding the solar system. They concluded that at its closest it could have passed just 0.58 light years from the sun.

Now there is more evidence for the argument.

Carlos and Raúl de la Fuente Marcos from the Complutense University of Madrid, and Sverre Aarseth from the University of Cambridge studied 340 objects that had very eccentric orbits compared to the other objects in the Oort cloud. Using simulations, the team used the mass, distance covered and position of these icy anomalies to work back to a time and location they may have had knocked off their courses by some common factor. The researchers concluded that Scholz star fit the bill nicely.

Coryn Bailer-Jones of the Max Planck Institute for Astronomy, Heidelberg had listed other large objects that have – or will – come by the solar system.

A depiction of the Oort cloud. Image: NASA

In his 2014 paper, engagingly titled ‘Close encounters of the stellar kind’ (which we’ve stolen here), the statistician and machine learning expert said there was a 90 percent probability that γ Microscopii that may have come as close as 1.1 light year away about 3.8 million years ago. Being about 2.5 times the mass of the sun it may have caused a larger effect on the solar system. It is now 229 light years away, in the Microscopium constellation visible in the southern hemisphere.

In about 1.3 million years, Gliese 710, now about 64 light years away in the Serpens constellation in the northern hemisphere, could come even closer – 0.3 light years. Since it is 60 percent of the mass of the sun, it can cause some significant disruption in the solar system.

But the closest a star could come is 0.04 light years. That will be Hip 85605, a star in the Hercules constellation in the northern hemisphere. Its current distance has been hard to gauge but has been put at between 18 to 28 light-years but it could pop over anywhere between 240,000 and 470,000 years. That could cause a worse disruption of the Oort cloud, potentially sending a spray of icy bodies into the inner solar system, including at Earth.

Because the Scholz star consisted of a red dwarf, when it passed by 70,000 years ago, it must have been hardly visible most of the time. But because it was – and is – magnetically active, it may have pulsed into visible brightness for minutes to hours at a time.
Our ancestors may have looked up in awe and wondered at this new mystery in the sky. But since this was around the time that humanity was flirting with extinction, they may have had other things to worry about.